what model ? I've not come across one that's switchable
[Edit: "oooh" at kitz chris's post]

My guess is that you previously had it set so the the mech saw you middle as the outer ring, granny as middle and had a "spare" click below that, but you've got your limit screws set so that the mech can't fall off the granny so all that could happen if you clicked down is a bit of loose cable

On fitting the new cables, you've either set it differently (so that the "up" click is tying to shift onto the bashring) or else you've just noticed what's always been going on with the spare click at the bottom

I simply set my 3 speed shifter used with my double/bash front so that the "Top" shift aligns to the "Middle" (larger of the two) ring position, the "Middle" shift position aligns to the "Granny" ring and then set the inner limit screw so that flicking down once more on the shifter does nothing other than make the cable go slack, not much point paying extra for a double shifter really...